1. Field of the Invention
This invention relates to the detector of electromagnetic radiation with a multiple quantum well (MQW) super-lattice structure, and more particularly to the sensing of long wavelength infrared radiation (LWIR) in the presence of background radiation from intrinsic events.
2. Description of the Related Art
The detection of LWIR with an MQW sensor has been reported in several publications: Levine et al., "Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors", J. Applied Physics Letters Vol. 64, No. 3, 1 Aug. 1988, pages 1591-1593; Levine et al., "Broad-band 8-12 .mu.m High-Sensitivity GaAs Quantum Well Infrared Photodetector", Applied Physics Letters Vol. 54, No. 26, 26 June 1989, pages 2704-2706; Hasnain et al., "GaAs/AlGaAs Multiquantum Well Infrared Detector Arrays Using Etched Gratings", Applied Physics Letters Vol. 54, No. 25, 19 June 1989, pages 2515-2517; Levine et al., "High-Detectivity D*=1.O.times.10.sup.10 cm .sqroot./W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector", Applied Physics Letters, Vol. 53, No. 4, 25 July 1988, pages 296-298.
The detector consists of a periodic heterostructure of GaAs quantum wells and AlGaAs barrier layers. The GaAs quantum well layers are doped with an n-type dopant such as silicon to provide electrons in the ground states of the wells for intersubband detection. Lacking an internal built-in electric field, the device operates as a photoconductive sensor. Under the influence of a bias voltage applied across n-doped contact layers at opposite ends of the MQW structure, electrons excited into a conduction band above the barrier heights flow through the device for collection at one of the contact layers. This majority current flow is sensed by an external ammeter and provides an indication of the amount of LWIR that is incident upon the device.
An improvement upon this detector is disclosed in pending U.S. patent application Ser. No. 07/457,613 filed Dec. 27, 1989 by Sato et al., "Dark Current-Free Multi-quantum Well Superlattice Infrared Detector", and assigned to Hughes Aircraft Company, the assignee of the present invention. In this improvement a blocking layer of barrier material is provided between the MQW superlattice and the collector contact. The blocking layer is substantially thicker than the barrier layers, thus eliminating most of the tunneling current component of the photodetector's dark current. This in turn allows the individual blocking layers to be made thinner, thus enhancing the detector's quantum efficiency.
Although the Sato et al. device improves upon the other detectors referenced above, all of these devices suffer from the presence of background noise signals caused by incident high energy photons or particles (collectively referred to hereinafter as radiation) from intrinsic events such as laser light, nuclear explosions and cosmic rays. Such intrinsic event radiation adds to the total majority carrier flow in the detector, resulting in spurious signals that can severely hamper the operation of the detector array, particularly in space-borne environments.